c Introduction. 



For a monograph upon the organization of the Braehiopoda, see 



Hancock, Phil. Trans., 1 858 ; see also Lacaze Duthiers, Comptes 



Kendus, ] 865, ii., p. 800. 

 For a monograph of the species Thecidium Mediterraneum, see Lacaze 



Duthiers, Ann. Sci. Nat., Ser. iv., tom. xv., i86t. 

 For the microscopic structure of the shell, see Carpenter, Palaeon- 



tographical Society's Memoirs, 1853, pp. 23-40. 

 For an account of a larval Brachiopod with figures, see Fritz Miiller, 



Reichert und Du-Bois Beymond^s Archiv., i860, p. 72, Taf. i., 



figs. I and 2. 

 See also pi. xi, fig. 2, and Description, pp. 232-234, infra. 



Class, Tunieata. 



Molluscoidea, which may he either solitary or social, either fixed 

 or free, but which are exclusively marine and are never aproctous. 

 The animals communicate with the exterior by two orifices, pierced 

 in a sacciform envelope, and here regarded as homologous with the 

 inhalant and exhalant siphons of the siphonate Lamellibranchiata. 

 As in those animals, the inhalant orifice brings into the organism 

 not only food and oxygen, but also spermatozoa, which find their 

 way to the ova by a route homologous to that described as taken 

 from the neural to the haemal surface of the gill, by the male 

 element in the Anodon (see pp. 64, 6^, infra) ; and consequently 

 over and along a structure which is regarded as the homologue 

 not of a dilated pharynx, but of the gills of the bivalves. The 

 external envelope of the Tunieata, from the external form of which 

 the name ' Ascidiae ' was given to the class by Savigny, is of very 

 various consistency, and of very various histological appearance ; 

 but it always secretes cellulose within its substance, and is made in 

 the way of conversion, and not in that of excretion as is the case in 

 other members of the sub-kingdom. In Tunieata as in Lamelli- 

 branchiata, the anus and generative ducts open into a space lined 

 by the internal tunic, more or less separable from the muscular 

 mantle ; but in the cloacal space of the Tunieata there is no pos- 

 terior adductor nor any specialized organ of Bojanus, though there 

 is contained in it their single nerve ganglion, which supplies parts 

 homologous with those supplied by the parieto-splanchnic of the 

 Lamellibranchiata. The heart of the Tunieata is ordinarily elon- 



